There are many actual and potential applications that benefit from monitoring and measurement of the physical conditions of various materials to determine the health and integrity of the underlying structure.
For example, an increasing number of aircraft are operating beyond their useful life expectancy. It is critical to ascertain which of their components are subject to strain, deflection, and growth of fatigue cracks, which dramatically reduce the integrity of the aircraft structures. The impact of multiple cracks on aircraft structural integrity depends not only on the local stress state, but is also strongly influenced by the crack pattern and crack geometries. To accurately predict the residual lifespan of affected service components, detailed characterizations of the cracks are absolutely essential.
As another example, in the past several decades, many researchers have been actively pursuing development of engineered sensor skins, defined as expansive flexible membranes densely embedded with distributed sensors. Sensor skins could enable an engineered system to approximate the self-protecting mechanism of bio-systems, as “feeling pain” provides early warnings to prevent further damage to an underlying structure. Other applications for sensor skins include wearable healthcare, telepresence, and aerodynamic monitoring.
Many different types of sensors have been developed to indirectly detect abnormalities and potentially damaging conditions based on their impact on the strain field, ultrasound wave propagation or vibration characteristics of a structure. Sensors using radio-frequency identification (RFID), piezoelectric materials and optical fiber sensors have been used extensively.
However, existing wireless sensors that have separate sensing units and wireless transmitting units transmit a sensor signal, usually in digital form, that has to be converted into a radio frequency (RF) signal to be transmitted. As a result, electrical power has to be supplied to such wireless sensors via an onboard power supply such as a battery, which limits the sensor's life span and increases cost, size and complexity. To overcome the power consumption problem, passive or self-powered sensors were proposed. The most popular passive wireless sensors are based on Surface Acoustic Wave (SAW), although these sensors are not very power efficient because of the double conversion of RF waves to elastic waves. Moreover, the substrate of a SAW sensor must be piezoelectric. Another type of passive wireless sensor utilizes an inductive coil antenna to broadcast a resonant frequency, a shift in which is caused by impedance changes induced by the measurand. However, sensors that utilize inductive coupling have a very limited range of operation due to high coupling losses. Crack detecting sensors that utilize piezoelectric materials have low power efficiency and operate in a limited range of temperatures. In addition, most of these sensors are point sensors and their spatial resolution is limited due to the limited number of sensors that can be deployed. Optical fiber based sensors provide a promising alternative that can provide distributed strain sensing for crack detection with good spatial resolution; however, optical fiber based sensors are expensive and delicate.